Police in Greater Manchester say a sixth patient has died after apparently being poisoned with insulin when saline drips were sabotaged at Stepping Hill hospital in Stockport last summer.

Earlier on Monday, a possible fifth victim was named as Linda McDonagh, 60, from Stockport. it was announced that John Beeley, 73, had also died after being poisoned. At least 21 people are believed to have been affected after drips were deliberately contaminated with insulin.

Tests are continuing to establish the precise cause of McDonagh's death because of a serious underlying medical condition – she had motor neurone disease. Detectives are trying to ascertain whether the poisoning contributed towards her death last week.

Officers have said they believe insulin played "more than a minimal part" in the deaths of three previously-named victims Tracey Arden, Arnold Lancaster and Derek Weaver, who died in summer 2011.

But they believe it is "highly likely" that the later deaths should be attributed to natural causes.

McDonagh's husband, Stewart, 61, said: "There's someone out there preying on vulnerable people while they should be in a place of safety."

She was diagnosed with MND two-and-a-half years ago and had been getting progressively weaker, her family said in a pooled interview with the Manchester Evening News. She was in hospital with pneumonia when she was apparently poisoned.

McDonagh's blood sugar levels "dropped dramatically" last summer while at Stepping Hill, her husband said. The saline drip was removed and replaced with another one, which "brought her back" and she recovered within minutes.

"The person who's done this to my wife and other people should come forward and search their conscience," he said. "They are in a hospital where they should be caring for people and helping them to get well. They should give themselves up."I feel quite sorry for the people at the hospital. They're all doing a good job. The staff there have been fantastic. I could not praise the hospital and health authority enough. It's a shame because this had cast a shadow over everyone there."

McDonagh recovered from pneumonia, but had a relapse and was taken back to the hospital on 7 January . She was put on a "non invasive ventilator" to keep her alive, with a mask over her face. McDonagh chose to remove the mask herself anddied on January 14.

Previously, she worked as an office administrator at an NHS clinic in Bramhall, Stockport.

Her daughter, Amanda Thorley, 29, said: "She was the best mother anybody could ever wish for, very patient and caring. She was very soft and gentle. She never complained even when she got poorly. She lived for every day. She was a new grandmother. She was very loving. She would do anything for anyone."

Her sister, Dr Rebecca Hildyard, 31, said: "She pretty much devoted herself to her family and her husband. Her career was her family. "

Thorley said her mother was tired and she'd had enough. "She didn't want to live that way anymore," she said. "She made her own decision. She chose to take the mask off and take her chances. It was amazing strength and courage - that was the part she could control," she added.

McDonagh was one of a number of patients who suffered a "hypoglycaemic episode" after the drips were contaminated on medical wards at the hospital in June and July 2011. The deaths of Arden, 44, Lancaster, 71, Weaver, 83, and William Dickson, 82, are being treated as suspicious.

The coroner for Manchester south, John Pollard, has been informedh, but no inquests will be held until the complex police investigation has been concluded.

Greater Manchester police said they had been made aware of the death of a 60-year-old woman at Stepping Hill hospital.

In a statement, the force said: "The woman, who was one of a number of patients who police believe suffered hypoglycaemic episodes between June and July 2011 after being administered contaminated products at Stepping Hill hospital, died in the early hours of Saturday. The coroner has been made aware and further tests are ongoing."

A spokesman for Stockport NHS foundation trust, which runs Stepping Hill hospital, said: "We are aware that a 60-year-old woman who was a victim of saline contamination last summer has died. Our thoughts are with her family and friends at this sad time."

The hospital has described the incidents as "deliberate and malicious acts of sabotage".

A nurse, Victorino Chua, 46, of Heaton Norris, Stockport, was arrested in connection with the investigation earlier this month. He was questioned on suspicion of three counts of murder and 18 counts of causing grievous bodily harm with intent. He is on police bail until April.

His arrest was prompted following an alleged incident overnight between 2 and 3 January, when notes were apparently altered and a patient was given extra medication. The patient has since been discharged after making a full recovery. Colleagues described Chua as a dedicated nurse who was very caring towards patients.

At the time, police stressed it was important not to jump to any conclusions.

Another nurse, Rebecca Leighton, 27, was arrested in July and held in custody for six weeks before the charges against her were dropped. She vehemently denied any wrongdoing and is understood to be appealing against her dismissal from the hospital after admitting stealing a prescription painkiller.

Police chiefs have previously stressed that detectives have not established the degree to which deliberate contamination of products may have contributed to the patients' deaths.

About 650 people have been spoken to by officers during the investigation at the hospital since the saline drip contamination was discovered.

The latest victim announced on Monday was one of 21 patients believed to have been deliberately poisoned.

The cases of a further 21 patients who suffered hypoglycaemic episodes are not being treated as suspicious.